No, not all hybrids are sterile, especially not in the plant world. Lions and tigers can crossbreed to form ligers (male lion + female tiger) and tigons (male tiger and female lion). Their habitats normally never intersect in the wild, so any breeding is accidental in zoos, or possibly intentional among people who breed exotic "pets".

Male ligers and tigons are sterile, but female ligers may breed with either lions or tigers to produce li-ligers or ti-ligers, although these are usually rather sickly and probably sterile. Hybrid vigor works well within a species, bringing in more variety into the genome (i. e. mongrel dogs are usually healthier than pure breeds), but not necessarily in the rare cases when interbreeding between closely related species is possible.

It hasn't been proved that H. sap. ever interbred with Neanderthals.

I always thought that passing beyond the rim meant becoming a being of energy like Shadows and Vorlons, as happened to humans eventually. Some were able to transform temporarily, others were so permanently. I wonder if there was ever a phase when Vorlons were mortal beings of flesh and blood, and if so, what that original form looked like (as they appeared different to every sentient being, except Centauri who saw nothing). For beings of energy rather than matter, the intergalactic void wouldn't pose a problem.

It's interesting to speculate that if Sinclair, as originally intended, had led B5 until the end (and presumably traveled into the past in Sleeping in Light rather than beyond the rim), and had had the relationship with Delenn, he would've had a child with his own descendant before going back to the past to sire the ancestors of that descendant! *my head hurts*

__________________"Summoned, I come. In Valen's name I take the place that has been prepared for me. I am Grey. I stand between the candle and the star. We are Grey. We stand between the darkness and the light."
- Delenn in Grey Council: "Babylon Squared"